This invention relates to the joining of bodies of material over bonding regions of large dimension using reactive composite materials such as reactive multilayer foils.
Reactive composite joining, such as shown in U.S. Pat. No. 6,534,194 B2 to Weihs et al and in U.S. Pat. No. 6,736,942 to Weihs et al. is a particularly advantageous process for soldering, welding, or brazing materials at room temperature. The process involves sandwiching a reactive composite material (RCM) between two layers of a fusible material. The RCM and the fusible material are then disposed between the two components to be joined, and the RCM is ignited. A self-propagating reaction is initiated within the RCM which results in a rapid rise in temperature within the RCM. The heat released by the reaction melts the adjacent fusible material layers, and upon cooling, the fusible material bonds the two components together.
Alternatively, depending upon the composition of the two components, the layers of fusible material are not used, and the reactive composite material is placed directly between the two components. Thermal energy released by ignition of the RCM melts material from the adjacent component surfaces and consequently joins the components.
Turning to FIG. 1, an arrangement 9 for performing the process of reactive composite joining of two components 10A and 10B is illustrated. A sheet or layer of reactive composite material 12 is disposed between two sheets or layers of fusible material 14A and 14B which, in turn, are sandwiched between the mating surfaces (not visible) of the components 10A and 10B. The sandwiched assembly is then pressed together, as symbolized by vise 16, and the reactive composite is ignited, as by match 18. The reaction propagates rapidly through the RCM 12, melting fusible layers 14A and 14B. The melted layers cool, joining the components 10A and 10B together. The RCM 12 is typically reactive multilayer foil, and the fusible materials 14A and 14B are typically solders or brazes.
The process of joining the two components 10A and 10B occurs more rapidly with a reactive composite joining process than with conventional joining techniques such as those which utilize furnaces or torches. Thus, significant gains in productivity can be achieved. In addition, with the very localized heating associated with the reactive composite joining process, temperature sensitive components, as well as dissimilar materials such as metals and ceramics, can be soldered or brazed without thermal damage. Fine-grained metals can be soldered or brazed together using a reactive composite joining process without grain growth, and bulk amorphous materials can be welded together with only a local excursion from room temperature, producing a high strength bond while minimizing crystallization.
The reactive composite materials 12 used in reactive composite joining process are typically nanostructured materials such as described in U.S. Pat. No. 6,534,194 B2 Weihs et al. The reactive composite materials 12 are typically fabricated by vapor depositing hundreds of nano-scale layers which alternate between elements having large, negative heats of mixing, such as nickel and aluminum. Recent developments have shown that it is possible to carefully control both the heat of the reaction as well as the reaction velocity by varying the thicknesses of the alternating layers. It has also been shown that the heats of reaction can be controlled by modifying the foil composition, or by low-temperature annealing of the reactive multi-layers after their fabrication. It is further known that alternative methods for fabricating nanostructured reactive multilayers include mechanical processing.
Two key advantages achieved by the use of reactive composite materials for joining components are speed and the localization of heat to the joint area. The increased speed and localization are advantageous over conventional soldering or brazing methods, particularly for applications involving temperature-sensitive components or components with a large difference in coefficient of thermal expansion, such as occur in metal/ceramic bonding. In conventional welding or brazing, temperature-sensitive components can be destroyed or damaged during the process. Residual thermal stress in the components may necessitate costly and time-consuming operations, such as subsequent anneals or heat treatments. In contrast, joining with reactive composites subjects the components to little heat and produces only a very local rise in temperature. Generally, only the adjacent fusible layers and the adjoining surfaces of the components are heated substantially. Thus, the risk of thermal damage to the components is minimized. In addition, reactive composite joining is fast and results in cost-effective, strong, and thermally conductive joints.
While conventional reactive composite joining works well in joining components over lengths less than about four inches and areas less than about 16 square inches, joining over larger lengths and areas presents particular challenges. It has been observed that for optimal joining it is advantageous that the surfaces to be joined be heated as uniformly, and as simultaneously, as possible. When the lengths and areas become larger, it is increasingly difficult to maintain the desired reaction simultaneity and uniformity from a single ignition point. In addition, larger joining region dimensions can exceed those of easily fabricated RCM's, requiring multiple pieces of reactive foil to cover the joint surface area. Even though the joining reaction spreads rapidly through the RCM, not every part of a large surface area joint area may be molten at the same time, possibly resulting in poor bonding between the components. Moreover, increasing the surface area to be joined presents increasingly stringent requirements for the uniform application of pressure to the components during the joining process.
Accordingly, it would be advantageous to provide a reactive composite joining process for use in joining components over surface areas which are larger than the size of a single sheet of reactive composite material, and which result in a strong and relatively uniform bond between the component materials.